Hi,
If I have understood your requirement correctly, then you can go for two different condition types (OR introduce one additional Condition Type) and have your condition record maintained for the condition types specific to the respective sales area.
That means Condition Type I would appear only for the Sales Area I.
& Condition Type II would appear only for the Sales Area II.
Still if your client insists to go for only one condition type for both the sales areas then you might have to go for Routines to have logic that the "Condition is If the sales area is I then the condition should be mandatory, Else Not mandatory".
Regards